Diffusion furnace hearth plate



Oct. 15, 1968 K. A. LANG 3,406,242

DIFFUSION FURNACE HEARTH PLATE Filed May 4, 1967 FIGJ INVENTOR.

K421. A. LANg Maafia 47' TOE/V5 rs United States Patent 3,406,242 DIFFUSION FURNACE HEARTH PLATE Karl A. Lang, 93 Rivo Alto Canal, Long Beach, Calif. 90893 Filed May 4, 1967, Ser. No. 636,154 4 Claims. (CI. 13-22) ABSTRACT OF THE DISCLOSURE A hearth plate for use in a diffusion furnace of the type in which the work to be heated is carried in a quartz tube inserted within an elongated heating coil seated within a helical groove provided in a surrounding sleeve made of refractory material, the hearth plate being interposed between the quartz tube and the sleeve and having complemental grooves to receive the adjacent portion of the heating 'coil, the lands between the grooves in the hearth plate and the sleeve thereby transmitting the weight of the tube, the work within the tube, and the hearth plate, so that the tube is kept from sagging without pressing against the heating coil.

CROSS REFERENCES TO RELATED APPLICATION A diffusion furnace of the type for which the present invention is particularly adapted is disclosed in my copending United States patent application, Ser. No. 447,343, filed Apr. 12, 1965, and entitled Furnace, now Patent Number 3,361,863, issued on Jan. 2, 1968.

BACKGROUND OF INVENTION Field of the invention The invention relates to diffusion furnaces and particularly to means for supporting the work-holding tube thereof to prevent sagging at the relatively high temperatures of operation of such a furnace.

Description of the prior art Diffusion furnaces have an important application in the processing of components such as transistors and the like, in which the operating conditions, particularly temperature, must be very precisely controlled. The type of diffusion furnace used for this purpose usually includes an elongated cylindrical casing disposed about a cylindrical sleeve of ceramic or like heat insulating, refractory material. A helical heating element or coil is supported within the sleeve, and seats within a helical groove provided in the sleeve.

A tube made of quartz or the like is insertable within the coil and carries the work to be processed. If the tube were without support between its ends, which are supported by structure of the furnace, the middle of the tube would tend to sag at the high operating temperatures employed. Consequently, the prior art practice is to use a ceramic sleeve which closely fits about the quartz tube along its length for support of the tube. The sleeve, in turn, is supported at its ends by the furnace structure and is spaced out of contact with the heating coil.

Unfortunately, the ceramic sleeve tends to fracture if extreme care is not exercised in bringing it up to temperature slowly, or in allowing it to cool. Replacement of the broken sleeve undesirably interrupts the operation, and obtaining replacement sleeves is somewhat expensive. Moreover, even if it does not fracture, the unsupported length of the sleeve itself tends to sag at extremely high operating temperatures.

SUMMARY The present invention relates generally to the use of one or more hearth plates interposed between the underside of the work-holding tube of a diffusion furnace and the heating coil carried by the sleeve which surrounds and supports the heating coil. The hearth plate thereby is adapted to support the tube along a substantial portion of its length. This eliminates the relatively expensive tube support sleeves of the prior art.

The present hearth plate is relatively inexpensive and, being open at the top for engagement with the tube, is characterized by much less susceptibility to thermal shock and breakage compared to an elongated, circumferentially continuous tube support sleeve. The hearth plate is not subject to sagging at elevated temperatures and therefore provides reliable support for the work-holding tube at all temperatures of operation of the dilfusion furnace. It preferably includes a plurality of adjacent recesses or grooves for receiving and seating the adjacent portion of the heating coil so that no significant weight is transmitted to the heating coil itself, which tends to soften slightly at elevated temperatures and would be damaged by flattening of its coil cross-section. Instead, the hearth plate transmits all loads directly to the sleeve which supports the heating coil.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a partial longitudinal cross-sectional showing of a diffusion furnace employing a prior art tube support sleeve to support the work-holding tube;

FIG. 2 is a partial longitudinal cross-sectional showing of the diffusion furnace of FIG. 1, slightly modified, and employing a hearth plate to support the work-holding tube according to the present invention;

FIG. 3 is a view taken along the line 33 of FIG. 2; and

FIG. 4 is a fragmentary perspective view of the hearth plate shown in FIGS. 2 and 3.

DESCRIPTION OF THE PRIOR ART AND THE PREFERRED EMBODIMENT Referring now to the drawings, FIG. 1 illustrates a portion of a difiusion furnace structure well known in the prior art. Only that portion of the furnace structure is illustrated which is pertinent to the present invention. More particularly, such a diifusion furnace structure includes a hollow cabinet 10 having side walls 12 and 14 which are provided with aligned transverse openings to accommodate the opposite end portions of a diffusion heating furnace 16. The cabinet 10 also includes a horizontally oriented tray 17 upon which the furnace 16 rests.

Other details of the furnace structure are not important to the present invention and therefore will be omitted for brevity.

The diffusion furnace 16 includes an elongated cylindrical metal casing 18 which is disposed about a cylindrical sleeve of refractory or ceramic insulating material 20. A helically arranged heating element or coil 22 is located within the hollow interior of the insulating sleeve 20. The sleeve 20 is preferably made in two parts, an outer cylindrical section 20a, and an inner cylindrical section 20 slidably fitted within the interior of section 20a. The coil 22 is seated within a helical groove provided in the inner surface of the section 20a of the insulating sleeve 20. In this manner, the heating coil 22 is supported in position within the sleeve 20 so that its heat is radiated inwardly within the hollow space defined by the coil.

A work-holding tube 24 made of quartz or the like is insertable within the heating coil 22, and is adapted to internally carry the work to be processed such as transistors or the like (not shown).

Since the quartz tube 24 tends to sag at the high operating temperatures of the furnace 16, the prior art structure shown in FIG. 1 includes an elongated support sleeve 26 made of ceramic or other refractory material which surrounds and closely receives the Work-holding tube 24. The ends of the support sleeve 26 extend beyond the opposite ends of the insulating sleeve and project through a pair of cylindrical, heat insulating end plugs or sleeves 28 and 30.

The end sleeves 28 and 30 fit against the adjacent ends of the central insulating sleeve 20, and are of a lesser diameter than the sleeve 20. Each of the end sleeves 28 and 30 is supported in position by the annular skirt of a short cylindrical element 36. Each of the elements 36 projects downwardly of ,the adjacent one of the aligned transverse openings in the cabinet walls 12 and 14, and includes a transverse outside flange which engages the outer surface of the adjoining cabinet wall. The inner end of the skirt of each element 36 presses against a sealing ring 34 of suitable heat insulation material, such as asbestos rope. Each ring 34, in turn, rests in sealing relation upon the outer lip of one of a pair of annular collars 32. Each collar 32 includes an inner lip which is secured to the adjacent end of the casing 18.

The cylindrical elements 36 and the end sleeves 28 and 30 are maintained in position by a pair of annular end plates 38, each of which overlies the flange of its associated cylindrical element 36 and the end of the associated end sleeve 28 or 30, as the case may be. A plurality of suitable screw fasteners 40 or the like secure the outer periphery of the end plates 38 to the cabinet walls 12 and 14, respectively.

As illustrated, both the ceramic support sleeve 26 and the work-holding quartz tube 24 are axially removable through the central openings in the end plates 38.

The sleeve 26 provides support for the quartz tube 24 along its length, but is itself subject to sagging at relatively high temperatures since it is supported only at its ends by the end sleeves 28 and 30. In addition, insertion of the support sleeve 26 with the quartz tube 24 is often accompanied by thermal shock and consequent breaking of the support sleeve 26. Replacement of the broken sleeve 26 undesirably delays the firing operation, and also raises the operating expenses.

Referring now to FIGS. 2 through 4, the diffusion furnace 16 illustrated is identical to that shown in FIG. 1 except that the central openings in the end sleeves 28 and 30 have been reduced in diameter to conform to the diameter of the quartz tube 24. In addition, the support sleeve 26 is completely eliminated and instead there is utilized an elongated hearth plate 42 according to the present invention.

The hearth plate 42 is made of a heat insulating or refractory material such as ceramic, and is generally arcuate in transverse cross-section. The plate 42 is interposed between the underside of the quartz tube 24 and the surrounding insulating sleeve 20, with its ends spaced slightly inwardly of the ends of the insulating sleeve 20. The arcuate inner surface of the plate 42 engages and supports the quartz tube 24 along substantially its entire length. The arcuate outer surface of the hearth plate 42 includes a plurality or series of recesses or grooves 44 which each have a track or orientation such that the grooves 44 complementally receive the adjacent turns of the heating coil 22, as best illustrated in FIG. 2. The arcuate curvature of the plate outer surface thus closely conforms to the curvature of the heating coil 22.

The grooves 44 are made sufficiently deep that the adjoining or intervening lands 46 defined by the grooves 44 rest upon the complemental, lands defined in the surrounding insulating sleeve 20 by the groove in the sleeve which accommodates the heating coil 22. Thus, the bottoms of the grooves 44 do not press against the heating coil 22, but instead the weight of the hearth plate 42 and the weight of the quartz tube 24 are transmitted by the hearth plate 42 directly to the insulating sleeve 20. Consequently, there is no tendency of the heating coil 22 to become flattened by such weight. In this regard, the heating coil 22 tends to soften slightly at the high operating temperatures of the furnace 16, and is susceptible to flattening if subjected to any weight.

The hearth plate 42 is open at its top and preferably extends laterally from the underside or lowest point of the quartz tube 24 only a short distance about the periphery of the tube 24. Consequently, the hearth plate 42 is not nearly so subject to thermal shock as the circumferentially continuous support sleeve 26 of the prior art. If desired, the hearth plate 42 may be made in sections disposed along the length of the insulating sleeve 20, which further reduces any tendency for breakage under thermal shock.

In summary, by utilizing the relatively inexpensive hearth plate 42, the quartz tube 24 is adequately supported along its length to prevent sagging at the extremely high operating temperatures of the furnace 16. The plate 42 remains in place during the life of the furnace, and use of the prior art support sleeve 26 is completely eliminated, along with the expense and delay attendant replacement of broken sleeves 26.

Various modifications and changes may be made with regard to the foregoing detailed description without departing from the spirit of the invention or the scope of the following claims.

I claim:

1. In a diifusion furnace of the type which is characterized by: a sleeve made of refractory material and having an inner surface which includes a helical groove defining adjoining lands; and an elongated heating coil seated in said groove and adapted to receive a tube made of quartz or the like for heating work located within said tube, the improvement comprising:

an elongated hearth plate made of refractory material and located between said heating coil and the underside of said tube and open at the top for engagement and support of said tube, and including means complement-a1 with said lands and in engagement therewith whereby the weight of said tube, said work, and said hearth plate are borne by said sleeve rather than by said heating coil.

2. The improvement according to claim 1 wherein the inner and outer surfaces of said hearth plate are arcuate in transverse cross-section to closely conform to the arcuate configuration of the adjacent said tube and said sleeve, respectively.

3. The improvement according to claim 1 wherein said means of said hearth plate are constituted by adjacent lands defined by a groove which is adapted to receive References Cited UNITED STATES PATENTS 2,294,034 8/1942 Jaeger 1325 XR 2,104,555 l/1938 Cousteix l3-25 XR 2,611,790 9/1952 Koch 1322 XR 3,083,445 4/1963 Hill l322 XR 3,170,060 2/1965 Oliver et al. 13-22 XR BERNARD A. GILHEANY, Primary Examiner.

H. B. GILSON, Assistant Examiner. 

